An orally active antiviral drug for the treatment of smallpox infection resulting from biowarfare or bioterrorism is needed as an alternative therapy for the estimated 40 million Americans who cannot be safely vaccinated. Although Cidofovir (CDV, Vistide(r)) inhibits poxvirus replication in cell culture and in mouse models, it must be administered by intravenous infusion and has shown a high level of nephrotoxicity. Novel, lipid ether conjugates of CDV have recently been described that inhibit smallpox replication in cell culture, and prevent mortality in mouse models of poxvirus infection after oral dosing. In addition, tissue distribution experiments indicate that the lipid-CDV conjugates are not deposited in the kidney, suggesting the possibility of diminished nephrotoxicity. This proposal includes the work necessary to choose a development candidate for the treatment of smallpox from two lead lipid-CDV conjugates (HDP-CDV and ODE-CDV), and to file an IND and conduct a Phase I clinical trial to assess the safety, tolerability and pharmacokinetics of this candidate. Specific aims and milestones that represent critical activities and key decisions in this proposal are: 1. Synthesize and characterize adequate drug substance to complete Aims 2 through 4. Characterization will include preformulation studies. Alternative routes of synthesis will also be examined. 2. Compare the pharmacokinetics and organ distribution of HDP-CDV and ODE-CDV in mice after oral dosing. 3. Compare the toxicological profiles of oral HDP-CDV and ODE-CDV in a 14-day dose range finding study in mice. 4. Compare the oral efficacy of HDP-CDV and ODE-CDV in mice infected with vaccinia, cowpox and ectromelia virus. The data generated in aims 1 through 4 will be used to choose which candidate to carry into full development (first milestone). At this point a pre-IND meeting will be requested with the FDA to discuss the proposed development plans. 5. Complete absorption, distribution, metabolism and elimination studies necessary to file an IND. 6. Produce cGMP drug substance for use in toxicology studies and Phase I clinical trials. 7. Conduct GLP safety pharmacology and toxicology studies necessary to file an IND. 8. Evaluate the efficacy of the lead compound in the cynomolgus monkey model of smallpox infection in collaboration with USAMRIID. Under the animal efficacy rule (Federal Register 67:37988-98, 2002), this study could provide the efficacy data necessary for FDA approval. 9. Manufacture prototype formulations, and produce cGMP clinical trials material. A Phase I protocol will be finalized in collaboration with the NIAID, and an IND will be filed with the FDA (second milestone). Upon FDA approval, a Phase I trial will be initiated to evaluate the safety, tolerability and pharmacokinetics of a single, escalating dose in human volunteers.